Disc drives are data storage devices that store digital data in magnetic form on a rotating storage medium on an information storage disc. Modern disc drives comprise one or more rigid information storage discs that are coated with a magnetizable medium and mounted on the hub of a spindle motor for rotation at a constant high speed. Information is stored on the discs in a plurality of concentric circular tracks typically by an array of transducers ("heads") mounted to a radial actuator for movement of the heads in an arc across the surface of the discs. Each of the concentric tracks is generally divided into a plurality of separately addressable data sectors. The recording transducer, e.g. a magnetoresistive read/write head, is used to transfer data between a desired track and an external environment. During a write operation, data is written onto the disc track and during a read operation the head senses the data previously written on the disc track and transfers the information to a host computing system. The overall capacity of the disc drive to store information is dependent upon the disc drive recording density. It is of particular importance in the disc drive art to maximize the disc drive recording density.
One of the most important parameters affecting the recording density of a disc drive is the spacing between the head and the magnetizable medium layer of the information storage disc, this spacing is known as the head media spacing. Closer head to media spacing allows for smaller magnetic signals, i.e., bits, recorded on the information storage disc which in turn allows for narrower track widths and consequent greater recording densities on the drive. As such, one way to maximize the disc drive recording density is to minimize head media spacing.
Head media spacing is dependent upon several factors, including: the head's "flying height," i.e., the physical separation distance between the top of the disc and the bottom of the recording head, the thickness of a lubricant layer, the thickness of a protective overcoat layer located on top of the magnetizable layer on the information storage disc, the thickness of a protective overcoat layer located on the air bearing surface of the head and the "any distance" that the recording transducers' pole tips are recessed below the level of the air bearing surface of the head.
Currently, efforts in the disc drive art have been centered on, among other things, decreasing the head media spacing by minimizing the thickness of the protective overcoat layers on the information storage disc and head. One major limiting factor on decreasing the protective overcoat layer thickness is the ability of the overcoat layer to protect the information storage disc and head from the build-up of corrosion products.
Corrosion causes corrosion products to build-up on the disc and head during the normal operating life of the drive. Corrosion products tend to accumulate on surfaces and interfere with the head's ability to fly over the disc surface. Corrosion occurs in the disc drive due to metals' propensity to be oxidized in the presence of oxygen or other oxidizing agents. The protective overcoat layer found on the information storage disc and head limits corrosion by eliminating the contact between the metal surfaces of the information storage disc and head with oxygen or other oxidizing agents found in the air.
It is also possible to limit corrosive product build-up on the disc and head by limiting the availability of oxygen within the disc drive. Here, the oxygen containing air within the disc drive can be replaced with a non-corrosive gas such as argon. The disc drive is then hermetically sealed so as to maintain a non-corrosive environment for the disc drive metal components. Within this non-corrosive environment the protective overcoat layers may be minimized in thickness, or in certain circumstances removed.
A major shortcoming of oxygen replacement within the drive is that the constrained non-corrosive gas within the hermetically sealed disc drive is unable to respond, i.e., expand or contract, to external changes in pressure or temperature. As is well known in the art, the volume of a gas is dependent upon its temperature and pressure. In general, a gasses volume is inversely proportional to the pressure applied to it and directly proportional to its temperature. For example, under ideal conditions an atmospheric pressure change from 101.3 kPa at sea level to 69.6 kPa at an altitude of 10,000 feet produces a volume increase of approximately 43% for an unconstrained constant mass of gas. The volume increase inside the disc drive results in increased pressure and has consequent affects on the alignment of components within the drive and potential failure of the seals within the drive. Against this backdrop the present invention has been developed.